1. Field of the Invention
The present invention relates to a diode pumped, high power, short pulse, single common reflective grating, chirped-pulse amplified laser (i.e., CPA) using an effective gain material, and more specifically, it relates to a method of producing holes or cuts with clearly differentiated sides and edges by employing such lasers.
2. Description of Related Art
High power laser systems (greater than 20W) with short pulses of the order of less than 10 picoseconds (ps) are of commercial interest for material processing. Such short pulse, (less than 10 ps) high power lasers have advantages over nanosecond pulse lasers for certain material processing applications. The mechanism for material removal is fundamentally different in the short pulse regime. In particular, less heat is dissipated into the surrounding substrate for a given energy and the subsequent removal of material is localized to the illuminated region. Therefore, short pulse lasers can produce very clean holes or cuts during material processing with clearly differentiated sides and edges.
An exemplary use in industry of such a system involves high precision machining of metals and alloys. Conventional mechanical lathes and machine tools are effective for cutting applications down to approximately 100 microns. Below this level, electron beam or laser tools are typically used for cutting or high precision machining (sculpting, drilling). Both electron beam and existing industrial laser technology remove material by a conventional thermal process where the material to be removed is heated to the melting or boiling point. The temperature of the surrounding material is determined by standard heat conduction from the region of interest. While small-scale features ( less than 100 microns) are readily achieved, they are often surrounded by a resolidified material (slag) and a significant heat-affected or shock zone often requiring post processing (e.g., annealing, electro-polishing, etc.). This heat-affected zone alters the properties of the material in the vicinity of the machined surface, often resulting in reduced material strength or modification of the composition of the material, particularly in the case of alloys.
A need still exists to reduce or essentially eliminate slag and/or heat-affected zones during material processing. There are various laser materials that have been incorporated into systems directed toward such needs. An exemplary system, such as Nd:YAG, can provide essentially the same power as that of the present invention, however, Nd:YAG systems are incapable of providing the needed short pulses due to a lack of bandwidth. Nd:Glass and Ti: Sapphire systems can provide the needed short pulses, but they have many inherent problems in providing sufficient average power.
Accordingly, the invention provides a high average power, short pulse laser system that uses end-pumped amplifiers in a chirped-pulse amplification architecture to produce pulses capable of material processing whose controlled output is directed to a workpiece.
The present invention uses a mode-locked laser source to generate at least 50 MHz for up to 8 ps, chirped pulses with sufficient power and TEM00 spatial mode structure. The output bandwidth of the oscillator is sufficient to support about a 1 ps pulse. The mode-locked laser source output is stretched using a common reflective diffraction grating contained in a pulse stretcher-compressor device, amplified by at least two amplifier stages, then recompressed by the common reflective diffraction grating stretcher-compressor device and directed by a beam delivery system to a desired workpiece. Between each laser subsystem, (usually computer based), a pointing and centering loop actively controls laser beam alignment.
A first pre-amplifier located after the stretcher is a linear-cavity regenerative amplifier (i.e., regen). The pre-amplifier, which can consist of multiple stages, amplifies the stretched pulses, for example, from a 50 MHz input pulse train and converts such a pulse down to 10 KHz, preferably 4 KHz, using switching of a cavity Pockels cell of the regen. The output of the regen is sent to a power amplifier that boosts the power up to greater than 20W.
The present system provides for a high average power, (greater than 20Wwith pulse-widths less than 2.5 ps), cost effective compact system which combines the functionality of a stretcher and compressor employing a single high precision reflective optical grating. The present laser system is capable of achieving desired power levels having necessary pulse-widths to perform high precision machining (e.g., holes or cuts with clearly differentiated sides and edges). The present invention converts the region to be removed from the workpiece from the solid-state to the plasma state so quickly that there is insufficient time for significant heat transfer beyond the depth of material removed. This results in the ability to perform extremely high precision machining of metals or alloys with essentially no slag (i.e., molten residue) or heat affected-zone and eliminates the need for cooling of the part during the machining process.